The present invention relates to a method of producing an Rxe2x80x94Fexe2x80x94B type sintered magnet, a method of preparing an alloy powder material for use as a raw material in the production of the Rxe2x80x94Fexe2x80x94B type sintered magnet, and a method of preserving the same.
A sintered magnet (permanent magnet) of a rare earth alloy is typically produced by compacting a powder of a rare earth alloy, sintering a compact of the powder obtained, and performing an aging heat treatment with respect to the sintered body. At present, two types of sintered magnets of rare earth alloys, which are a samarium-cobalt type magnet and a neodium-iron-boron type magnet, are used widely in different fields. Of the two types, the neodium-iron-boron type magnet (hereinafter referred to as xe2x80x9cRxe2x80x94Fexe2x80x94B type magnetxe2x80x9d where R is one of rare earth elements inclusive of Y, Fe is iron, and B is boron) has been applied positively to various electronic equipment because of its highest magnetic energy product among various magnets and relatively low cost. The Rxe2x80x94Fexe2x80x94B type rare earth alloy consists of a main phase mainly composed of an R2Fe14B tetragonal compound, an R-rich phase composed of Nd and the like, and a B-rich phase. It is to be noted that Fe may be partly replaced by a transition metal such as Co or Ni. As documents disclosing Rxe2x80x94Fexe2x80x94B type rare earth sintered magnets to which the present invention is applied appropriately, U.S. Pat. Nos. 4,770,723 and 4,792,368 are incorporated by reference in the present specification.
To produce a rare earth alloy forming such a magnet, there has conventionally been used ingot casting whereby a molten metal alloy as a raw material is placed in a mold and cooled relatively slowly. An alloy ingot produced by ingot casting is powdered by a well-known pulverizing process. The alloy powder thus produced is compacted by various powder pressers and transported into a sintering furnace, where it is subjected to a sintering process.
In recent years, attention has been focused on quenching methods represented by strip casting and centrifugal casting, whereby a solidified a alloy thinner than an ingot (hereinafter referred to as xe2x80x9can alloy flakexe2x80x9d) is formed from a molten metal alloy by bringing the molten metal alloy into contact with a single roll, double roll, a rotating disk, or an inner side of a rotating cylindrical mold and thereby performing relatively rapid quenching. The thickness of an alloy piece produced by such a quenching method is normally in the range of about 0.03 mm to about 10 mm. In accordance with a quenching method, the molten metal alloy begins to solidify from a surface thereof in contact with a cooling roll (roll contact surface and a crystal grows from the roll contact surface in the direction of thickness into a columnar configuration. As consequence, a quenched alloy produced by strip casting or the like has a structure including an R2Fe14B crystal phase which has a size not less than about 0.1 xcexcm and not more than about 100 xcexcm in the direction of a minor axis and a size no less than about 5 xcexcm and not more than about 500 xcexcm in th direction of a major axis and an R-rich phase which is present dispersively in a grain boundary of the R2Fe14B crystal phase. The R-rich phase is a nonmagnetic phase containing a rare earth element R at a relatively high concentration and having a thickness (corresponding to the width of the grain boundary) of about 10 xcexcm or less.
Since a quenched alloy has been quenched in a shorter period of time (cooling rate: not less than 102xc2x0 C./sec and not more than 104xc2x0 C./sec) than an alloy (ingot alloy) produced by the conventional ingot casting (die casting), it features a miniaturized structure and a reduced crystal particle size. The quenched alloy also has the advantage of the R-rich phase with excellent dispersion since the grain boundary occupies a large area and the R-rich phase is spread widely in the grain boundary. These features allow a magnet having superior magnetic properties to be produced by using the quenched alloy.
In the present specification, blocks of a solidified alloy obtained by quenching or cooling a molten metal will be termed xe2x80x9calloy blocksxe2x80x9d which include solidified alloys in various forms such as the alloy ingot obtained by the conventional ingot casting and the alloy flake obtained by a quenching method such as strip casting. An alloy powder subjected to compacting is obtained by grinding the alloy blocks into a coarse powder (having an average particle size of, e.g., 10 xcexcm to 500 xcexcm) by, e.g., hydrogenation pulverizing (i.e., hydrogenation occlusion and/or various mechanical grinding methods and then fine pulverizing the coarse powder.
However, an alloy powder produced by a quenching method, which is represented by a strip cast alloy, has the problem of susceptibility to oxidation. In general, a powder of a rare earth alloy is susceptible to oxidation and has a risk of heat generation or ignition. A powder of a quenched alloy is considered to have a particularly high risk of heat generation or ignition since the R-rich phase susceptible to oxidation easily appears on a surface of a powder particle of the quenched alloy.
To circumvent the problem, e.g., Japanese Patent Publication No. 6-6728 (Applicant: Sumitomo Special Metals Co., Ltd., Filing Date: Jul. 24, 1986) discloses a method of forming a thin oxide film on a surface of a powder of a rare earth alloy. The publication also discloses that, in order to provide superior magnetic properties, the average particle size of the powder of the rare earth alloy subjected to compacting is preferably in the range of 1.5 xcexcm to 5 xcexcm. If the average particle size is smaller than 1.5 xcexcm, the proportion of the oxide becomes excessively high so that the magnetic properties are degraded. If the average particle size is larger than 5 xcexcm, magnetization inversion easily occurs to reduce a coercive force. Japanese Patent Publication No. 6-6728 is incorporated by reference in the present specification.
To improve the compressibility (compactibility) of a powder of a rare earth metal, on the other hand, the specification of U.S. Pat. No. 5,666,635 (Assignee: Sumitomo Special Metals Co., Ltd.) discloses a technique for producing a fine powder having an average particle size of 1.5 xcexcm to 5 xcexcm by adding and mixing a 0.02 wt % to 5.0 wt % lubricant prepared by liquidizing at least one fatty acid ester in a coarse powder of an alloy for an Rxe2x80x94Fexe2x80x94B type sintered magnet having an average particle size of 10 xcexcm to 500 xcexcm and milling the mixture in a jet mill by using an inert gas. U.S. Pat. No. 5,666,635 is incorporated by reference in the present specification.
As a result of conducting a study, however, the present inventor has encountered the problem that, even if the conventional technique is used, cracks or hips assumedly resulting from poor compressibility during compacting are likely to be produced in a compact of the alloy powder. The problem was particularly notable when a rare earth alloy powder having a relatively sharp particle size distribution from which the smaller and larger particle sides of the rare earth alloy powder had been removed was used.
The present invention has been achieved to solve the foregoing problem and a primary object of the present invention is to provide a method of producing an Rxe2x80x94Fexe2x80x94B type sintered magnet and a method of preparing an alloy powder material for the Rxe2x80x94Fexe2x80x94B type sintered magnet which can reduce cracks and chips in a compact by improving the compactibility, particularly compressibility, of the alloy powder material for the Rxe2x80x94Fexe2x80x94B type sintered magnet and thereby improve productivity.
In the present specification, a powder composed only of a rare earth alloy (including an oxide film formed through the oxidation of a surface of a rare earth alloy powder) will be termed xe2x80x9ca rare earth alloy powderxe2x80x9d and a rare earth alloy powder having a particle surface coated with a lubricant will be termed xe2x80x9ca rare earth metal alloy powder materialxe2x80x9d. The xe2x80x9crare earth alloy powder materialxe2x80x9d may contain an excess of lubricant in addition to the lubricant coating the surface of the rare earth alloy powder and, if necessary, may further contain a binder.
As a result of conducting various studies in view of the foregoing problem presented by the conventional technology, the present inventor has assumed after the preparation of an alloy powder material composed of a rare earth alloy powder having a surface coated with a lubricant and before compacting, the content of the lubricant in the alloy powder material varies and the resulting variations in (and/or uniformity of) the content of the lubricant are related to the compressibility of the powder material. The variations in the content of the lubricant results in cracks or chips in a compact of the powder material.
The present inventor has further conducted a study and achieved the invention based on the finding that the compressibility can be improved and the occurrence of cracks and chips in the compact can be reduced by reducing, prior to the compacting of the alloy powder having the surface coated with the lubricant, the lubricant contained in the alloy powder material to a specified amount or less through evaporation.
A method of producing an Rxe2x80x94Fexe2x80x94B type sintered magnet according to the present invention includes the steps of: (a) preparing an alloy powder material in a first state in which a lubricant, in an amount equal to or more than a first amount, has been applied to a surface of an alloy powder for the Rxe2x80x94Fexe2x80x94B type sintered magnet; (b) partially evaporating the lubricant in the alloy powder material in the first state and thereby preparing the alloy powder material in a second state in which the amount of the lubricant has been reduced to a second amount or less; (c) compacting the alloy powder material in the second state and thereby forming a compact; and (d) sintering the compact.
In an embodiment, the step (a) may include the step of fine milling a coarse powder of the alloy while supplying the lubricant.
In another embodiment, the step (a) may include the step of mixing the alloy powder with the lubricant, while supplying the lubricant to the alloy powder prepared in advance.
Preferably, the step (b) includes the step of allowing an inert gas to flow in an airtight container containing therein the alloy powder material in the first state.
After the step (b), the method may further include the step of preserving the alloy powder material in the second state, while allowing the inert gas to flow in the container or in another airtight container.
The method may further include the step of sampling the alloy powder material in the second state reserved in the container and analyzing a composition of the sampled alloy powder material, wherein the step (c) is performed after the sampling and analyzing step.
Preferably, the alloy powder has an average particle size in the range of 3 xcexcm to 6 xcexcm.
The alloy powder having a specific surface area in the range of 0.45 to 0.55 m2/g when measured by the BET method an be used appropriately.
Preferably, the first amount is equal to or more than 0.15 wt % of a weight of the alloy powder.
Preferably, the second amount is equal to or less than 0.12 wt % of a weight of the alloy powder.
As the lubricant, a lubricant containing a fatty acid ester as a main component can be used.
In the step (a), the lubricant diluted with a solvent may be applied to the surface of the alloy powder. The lubricant diluted with the solvent may be supplied in the step of fine milling the coarse powder of the alloy or may be mixed in the alloy powder prepared in advance. Preferably, the total amount of the solvent and the lubricant contained in the alloy powder material in the second state is equal to or less than 0.5 wt % of a weight of the alloy powder. As the solvent, a petroleum solvent can be used. In this case also, a lubricant containing a fatty acid ester as a main component can be used as the lubricant.
In accordance with another aspect of the present invention, there is provided a method of preparing an alloy powder material for an Rxe2x80x94Fexe2x80x94B type sintered magnet, the alloy powder material being formed of an alloy powder having a surface to which a lubricant has been applied.
The method of preparing the alloy powder material according to the present invention includes the steps of: (a) preparing the alloy powder material in a first state in which the lubricant in an amount equal to or more than a first amount has been applied to a surface of the alloy powder for the Rxe2x80x94Fexe2x80x94B type sintered magnet; a (b) partially evaporating the lubricant in the alloy powder material in the first state and thereby preparing the alloy powder material in a second state in which the amount of the lubricant has been reduced to a second amount or less.
In an embodiment, the step (a) includes the step of fine milling a coarse powder of the alloy, while supplying the lubricant.
In another embodiment, the step (a) includes the step of mixing the alloy powder with the lubricant after the step of fine milling.
Preferably, the step (b) includes the step of allowing an inert gas to flow in an airtight container containing therein the alloy powder material in the first state.
Preferably, the alloy powder has an average particle size in the range of 3 xcexcm to 6 xcexcm.
The alloy powder having a specific surface area in the range of 0.45 to 0.55 m2/g when measured by the BET method can be used appropriately.
Preferably, the first amount is equal to or more than 0.15 wt % of a weight of the alloy powder.
Preferably, the second amount is equal to or less than 0.12 wt % of a weight of the alloy powder.
As the lubricant, a lubricant containing a fatty acid ester as a main component can be used.
In the step (a), the lubricant diluted with a solvent may be applied to the surface of the alloy powder. The lubricant diluted with the solvent may be supplied in the step of fine milling the coarse powder of the alloy or may be mixed with the alloy powder after the step of fine milling. Preferably, the total amount of the solvent and the lubricant contained in the alloy powder material in the second state is equal to or less than 0.5 wt % of a weight of the alloy powder. As the solvent, a petroleum solvent can be used. In this case also, a lubricant containing a fatty acid ester as a main component can be used as the lubricant.
In accordance with still another aspect of the present invention, there is provided a method of reserving an alloy powder material for an Rxe2x80x94Fexe2x80x94B type sintered magnet.
The method of preserving the alloy powder material according to the present invention includes the step of: preserving, in an airtight container in which an inert gas is allowed to flow, the alloy powder material composed of an alloy powder for the Rxe2x80x94Fexe2x80x94B type sintered magnet having a surface to which a lubricant in a specific amount or less has been applied.
Preferably, the alloy powder has an average particle size in the range of 3 xcexcm to 6 xcexcm.
The alloy powder having a specific surface area in the range of 0.45 to 0.55 m2/g when measured by the BET method can be used appropriately.
Preferably, the specified amount is equal to or less than 0.12 wt % of a weight of the alloy powder. As the lubricant, a lubricant containing a fatty acid ester as a main component can be used.
The alloy powder material may contain the lubricant and a solvent. In this case, the total amount of the lubricant and the solvent is preferably equal to or less than 0.5 wt % of a weight of the alloy powder. As the solvent, a petroleum solvent can be used. In this case also, a lubricant containing a fatty acid ester as a main component can be used as the lubricant.